This invention relates to a method and apparatus for determining the distance of a mobile wireless device (xe2x80x9cmobilexe2x80x9d) from a base station in a timing advance controlled system.
Systems of remote devices, e.g., mobiles that communicate with a central device over a common channel in a TDMA fashion, commonly rely on setting a timing advance (TA) for each of the remote devices. The timing advance scheme is necessary to better utilize the several timeslots at which each mobile transmits, i.e., to assure that the remote transmitted packet is received neither too early nor too late at the central device. The TA allows the remote devices to move about in relation to the central device and causes those devices that are farthest to transmit with minimal or no delay, while those that are closest transmit with the maximal delay, avoiding overlap of the received transmissions at the central device.
For example, in a Global System for Mobiles (GSM) system, the specification (see ETSI/TC SMG Recommendation GSM 5.10 V.3.5.1 published October 1992) allows for mobiles to transmit from a distance of 35 km (which would have a timing advance setting of 63 units or bits). The nearest mobile station operates with an advance setting of zero units. The central device or base transceiver station (BTS) and mobiles communicating with it are time synchronized. Absent the use of the timing advance, the packet sent by mobiles near the BTS has the risk of overlapping any uplink packets from the cell fringe by 63 units of delay, where the mobile at the cell fringe transmits in the timeslot immediately prior to the near mobile transmission.
The best accuracy that can be obtained for distance is entirely dependent on the size of the unit of the timing advance. The distance radio waves propagate in the 3.71 microsecond bit duration is 1100 m. Since the timing advance indicates the delay to be used after a reference signal of the BTS is received at a mobile, the actual distance between changes in the TA for the mobile is 1100 m/2, or 550 m, which accounts for half the delay in the roundtrip.
It is an object of the present invention to provide a manner to successively approximate the distance of a mobile more accurately than that provided by prior art timing advance settings.
It is an object of the present invention to provide a method to use existing timing advance systems and interpose a programmable delay to accelerate the occurrence of a timing advance command.
It is an object of the present invention to provide a method to determine the distance of a mobile that causes minimal bit overlaps of the tested mobile packets and other packets of mobiles operating under timing advance control with the BTS.
To overcome the limitations in the prior art described above and to overcome other limitations of the prior art that will become apparent upon reading and understanding the present specification, the present invention discloses a method to determine a distance modifier between a mobile transmitter and a central transceiver in a timing advance controlled system having a timing advance including determining if a timing advance changes, and provided it does not, adding progressively larger delays to delay signals of the mobile received at the transceiver. When the timing advance changes, the distance modifier of the mobile based on the size of the last delay is calculated.
In an embodiment of the invention, the method and apparatus is implemented in a time division multiple access (TDMA) system that has a timing advance feature for synchronizing the transmissions of mobile stations to a base station. The timing advance allows the base station to command the mobile station to delay or advance a transmission in time one or more timing advance delay periods (TA) for synchronization purposes. According to the embodiment, the base station also has capability to adjust a received signal (burst) in increments that are less than the TA period to effect an artificial delay, for processing after the burst is received.
The process performed in the embodiment according to the method and apparatus is active and available in the base station when the base station is powered up. When it is desired to determine the distance of the mobile station from the base station, for example, as part of a 911 call mobile location function, a step delay, which is less than the TA, is added to a burst received from the mobile station. The burst having the step delay added is then processed. In the processing, a determination is made as to whether or not the addition of the step delay causes a TA change criteria to be satisfied for the mobile station i.e., whether actual delay of the amount added by the step delay would cause the base station to command the mobile to perform a timing advance change. If the TA change criteria are satisfied, a second processing stage is begun. If the timing advance change criteria for the mobile station are not satisfied for the first processed burst, the step delay is successively and cumulatively added to subsequent received bursts transmitted by the mobile station until the TA change criteria are satisfied. When the TA change criteria are satisfied, the second processing stage is begun.
In the second processing stage of the embodiment, subsequently received bursts from the mobile station are delayed by a base delay equal to the total number of first step delays needed to cause the TA criteria to be satisfied minus one first step delay. This base delay is then cumulatively adjusted by a second step delay for subsequent bursts received from the mobile. The second step delay is smaller than the first step delay. Each burst is processed and a determination is made as to whether the TA change criteria are satisfied after a second step delay adjustment. The second stage processing is repeated until the TA change criteria are not satisfied. When it is determined that the TA change criteria are not satisfied, the distance of the mobile from the base station may be calculated. The distance is calculated using the TA delay period, the number of first step delays needed minus one, and the number of second step delays used in the second stage processing. This gives an accuracy dependent on the size of the second step delay, which is more accurate than the accuracy obtainable from using TA delay alone.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects.